1. Field of the Invention
This invention relates generally to electric discharge devices and more particularly the invention relates to such devices used in air sterilization.
2. Description of the Related Art
One type of electric discharge device utilizing mercury vapor is provided thermionic electrodes and has a starting gas therein such as argon, krypton, xenon, or neon. Such devices are efficient sources of visible light and emit in addition ultraviolet light which is useful in therapy, photo-chemistry, sterilization, irradiation of foodstuffs or the like. When desired, a luminescent material, such as a fluorescent or a phosphorescent material, is associated with the device to transform the ultraviolet light into visible light which complements and supplements the visible light emitted by the discharge in the device.
Typically, an electric discharge device has two primary parts, the tube and the base. The tube is commonly embodied as fluorescent light bulbs and tubes. Other common embodiments are ultraviolet tubes used in tanning booths, and grow-lamps for plants. These tubes are made in standard sizes and power capabilities, and have standardized outputs. Tubes having both elongated forms and bulb form are well-known in the art.
The base is designed so that the tube may be easily installed and removed, while holding the tube firmly during other times. The base typically incorporates a ballast. The ballast is an electrical component that converts a standard power input (i.e. 117 V, 0.10 A AC household current) to a form appropriate to the tube.
In certain types of electric discharge devices the energy input and the heat dissipating characteristics are in such relation that the envelope is at an elevated temperature and the gaseous vapor therein is at an elevated pressure, such as atmospheric pressure, during the operation of the device.
Container glasses which are highly heat resistant, chemically inert with respect to the hot, ionized metal vapor and which transmit ultraviolet light are desirable for use in such devices. Glasses which have a high transmission characteristic for ultraviolet rays typically age rapidly when subjected to such rays of shorter wave length. Further, these ultraviolet transmitting glasses typically have too low a softening temperature and are not inert to the hot mercury vapor and to other hot metal vapors which may be present in the lamp, such as cesium, cadmium, or zinc vapor. On the other hand, glasses which are inert to such metal vapors and which have a high softening temperature typically do not transmit enough ultraviolet at the thicknesses required in lamp containers to make the lamp useful as an ultraviolet generator.
Ultraviolet tubes are generally filled with mercury vapor. When an electrical potential difference is created across the tube, the mercury atoms are excited and emit ultraviolet radiation. The amount of radiation emitted is generally proportional to the power applied. Therefore, it is expected that a more powerful ultraviolet tube results, given a constant voltage, by simply increasing the current of the device.
However, as the power is increased, so too increases the heat produced. If too much heat is produced, the tube may enter a thermal runaway condition, resulting in decreased tube life. Therefore, the heat produced as a side effect practically limits the current of such tubes to 400 mA. This limit on the current also then limits the radiation output.
Another problem in electric discharge devices is skin-effect cooling. In skin-effect cooling, air moving across the outside of a tube causes the ultraviolet output level to drop. It is known that if the ambient temperature drops below 72.degree. F. or air is blown over the tube, ultraviolet output drops at an extremely rapid rate --as much as a 75% depreciation of output at approximately 58.degree. F.
An additional problem of electric discharge devices has been ozone creation. The energy below 230 nm radiated by these tubes apparently leads to the creation of ozone outside the tube.
Although it has long been known that electric discharge devices may be used for air sterilization, their actual implementation has been fairly limited. One reason for this has been some of the technological difficulties already discussed. Skin-effect cooling is one stumbling block in using electric discharge devices in many applications such as air sterilizers. Because of skin-effect cooling, 2 to 4 times as many tubes must be used for equal output, and the tubes must be replaced every 2500 hours. The high cost of extra tubes and the maintenance costs have limited the practicality of ultraviolet air sterilizers. One common usage of electric discharge devices as air sterilizers is in operating rooms and hospitals. However, higher output tubes used in these applications produce large amounts of ozone.
Table 1 shows the dosage of 254 nm ultraviolet radiation necessary to kill several common air contaminants. As can be seen, a dose of at least 11 .mu.W/cm.sup.2 is necessary to sterilize air containing these contaminants. However, the kinds of devices used for this purpose are very limited. Such devices cannot be used in such general purpose applications as air sterilization in smaller systems such as in automobiles and in trucks. Nonetheless, these other areas suffer from the same kinds of problems as hospital operating rooms. That is, they suffer from germs, mildew and odors.
This problem has become especially acute in vehicles. Vehicle air conditioning systems have become very efficient. However, this efficiency has led to very damp evaporator coils and drip pans of these air conditioner systems. Such damp areas become breeding grounds for mold and fungi, which emit foul odors and can even lead to disease. Typically, vehicle air conditioners are treated with chemicals to reduce or eliminate the mold and fungi. However, these chemicals have only temporary effect--typically less than one year, and must be reapplied. Furthermore, use of such chemicals may have harmful environmental and human-health side-effects. Use of the chemicals is inconvenient since application must be made directly to the evaporator coil and drip pan--parts which are typically not readily accessible.
Electric discharge devices have not been considered for removal or elimination of mold and fungi that cause odors growing in or around the evaporator coil of vehicle air conditioners. One reason is that the electrical system of a vehicle has not been well suited as a supply to electric discharge devices. A second reason is the harsh operating environment. Vehicle air conditioning systems typically operate at 20.degree.-30.degree. F. Electric discharge device output usually drops to near 0 at 20.degree. F. The interior of the vehicle may be well over 140.degree. F. This broad operating range has generally been unsuitable for electric discharge devices. A third reason is that the compact design of vehicle air conditioning systems typically do not allow for the addition of bulky electric discharge devices. A fourth reason is that the physical instability of vehicles--bumps, jars, shakes--are generally considered poor sites for electrical discharge devices. A fifth reason is that the electric discharge device typically create large E-fields and emit large amounts of RF. The E-fields and the RF interrupt the operation of other electronic devices.
TABLE 1 ______________________________________ Dose Dose Bacteria (.mu.W/cm.sup.2) Bacteria (.mu.W/cm.sup.2) ______________________________________ Bacillus anthracis 45 Pseudomoneas 35 B. megatherium 11 fluorescens (veg.) Salmonella 40 B. megatherium 27 enteritis (spores) S. typhosa- 22 B. parathyphosus 32 Typhoid fever B. subtilis 70 S. paratyphi- 32 (spores) 120 Enteric fever Clostridium tetani 130 S. typhimurium 80 Corynebact. 34 Sarcina lutee 197 diptheriae Serratia 24 Eberthella typhosa 21 marcescens Escherichia coli 30 Shigella 22 Leptospira Spp.- 32 dysenteriae Infectious jaundice Dysentery form Micrococcus 61 Shigella flexneri 17 candidus Dysentery form Micrococcus 81 Shigella 17 piltonencis paradysenteriae Micrococcus 100 Spirillum rubrum 44 sphaeroides Staphylococcus 18 Mycobacterium 62 albus tuberculosis Staphylococcus 26 Neisseria catarrphalis 44 aureus Phytomonas 44 Streptococcus 22 tumefaciens hemolyticus Proteus vulgaris 26 Streptococcus 62 Pseudomonas 55 lactis aeruginosa Streptococcus 20 Yeasts viridans Common yeast cake 60 Mycobacterium 100 Saccharomyces 60 tuberculi ellipsoideus Vibrio 34 (Bakers yeast) comm-Cholera Saccharomyces 60 Various algae cerevisiae Diatoms (Bakers yeast) Green algae 3600-6000 Torula sphaeric 23 Blue algae (as found Worms in milk and cream) Nematode eggs 400 Protozoa Paramecium 640-6000 ______________________________________